The present invention relates to a light-weight material containing expanded perlite, alkali silicates, optionally added hardeners for aqueous alkali silicates, and processes for producing same.
Light-weight materials containing expanded perlite are the subject matter of Applicant""s WO 96/04217. They are prepared by activating silicate light-weight aggregates, such as perlite, by an alkaline earth hydroxide and a little water, followed by hydrothermal treatment with steam to bond the particles to one another.
This method is -time-consuming, and also relatively energy-consuming because of the hydrothermal treatment. Thus, there is a need for a more rapid, more simple and less energy-consuming process for the preparation of a light-weight material.
From DE-A-28 13 745, a flexurally rigid solid composite material is known which is obtained by heating from 20 to 50 parts by weight of expanded perlite in the form of a finely milled powder with from 9.5 to 19 parts by weight of sodium silicate or potassium silicate, from 2 to 9 parts by weight of zinc oxide, and from 21.5 to 67 parts by weight of water, including the water content of the silicate. The mixture preferably also comprises a solidifying agent, such as sodium fluorosilicate. The material is preferably shaped in the form of half shells with grooves and tongues, being thus suitable for the heat insulation of pipings. As an advantage of this material, it is stated having a smooth, aesthetically appealing surface and a high resistance to oil and hot water. The admixed material, after an interim storage of up to 2.5 hours, is heated, wherein microwave energy may also be used in addition to slow heating from outside. It is prepared by casting into molds, followed by pressing in a hydraulic press. The hardening may also be effected by the pressing in of carbon dioxide rather than using sodium fluorosilicate.
A further development of this material is described in EP-A-0 048 570, the flexural strength being increased by the addition of phosphates or clays.
From the Derwent referate 93-218464/27, a heat-insulating product is known which consists of a mixture of expanded perlite and water glass as well as a hydrophobizing agent; it is treated with carbon dioxide for hardening. The product contains 60% by weight of expanded perlite and 40% by weight of water glass, and an organosilicate hydrophobizing agent. The compacting factor is 2.5, corresponding to a volume reduction to 40%. The compressed system must be gassed with CO2, followed by drying at ambient temperature with circulating air for 24 to 48 hours. This is followed by hardening at 300 to 450xc2x0 C. in several cycles in a CO2 atmosphere.
From DE-C-44 38 627, an insulating and levelling composition is known which is constituted of 70% by volume of expanded perlite and 30% by volume of light-weight granules, such as pumice or light-weight concrete. This blend is mixed using a forced-circulation mixer and alkali water glass. The resulting product is packed in sacks and employed at the building site where it is compacted by 20 to 30% and hardened in the air. This material has a poor mechanical strength. Another drawback of this material is the fact that it often quickly hardens within the sack already upon application of mechanical pressure.
It has been the object of the present invention to provide a fiber-free inorganic non-combustible water-repellent light-weight material based on expanded perlite, alkali silicates and optionally added hardeners for aqueous alkali silicates, which material is easily and simply prepared, does not require any toxic chemicals and can be processed simply into sheets, molded parts or blocks suitable for heat insulation, especially of walls and buildings.
This object has now been achieved by using expanded perlite having a grain size of from 0.8 to 6 mm in the presence of a hydrophobizing agent, wherein bulk densities in the range of from 60 to 500 kg/m3 are achieved. The content of expanded perlite is preferably increased to at least 50 mass percent. In contrast to the prior art, the expanded perlite is not milled to a powder having a maximum grain size of 0.3 mm, but it is possibly used in an intact condition with a grain size of from 0.8 to 6 mm, preferably with a grain size in the range of from 0.8 to 3.5 mm.
Sodium water glass with a weight modulus of from 2.0 to 4.0, preferably from 2.5 to 3.7, or a potassium water glass with a weight modulus of from 1.0 to 3.0, preferably from 1.8 to 2.7, are preferably used as the alkali silicates. Mixtures of different sodium and potassium water glasses may also be used.
It is essential to the invention that the alkali silicates used as binders contain a hydrophobizing agent. In particular, alkyl siliconates have proven useful as said hydrophobizing agents. In principle, however, other typical hydrophobizing agents may also be used, provided they are soluble in the water glass and alkali-resistent and thus result in a water-repellent bulk hydrophobization with a w value of less than 0.5 kg/m2h0.5.
Conventional liquid or solid hardeners may be used as the hardener for aqueous alkali silicates. Typical suitable hardeners ers include zinc, aluminum, aluminate and silicate containing preparations, ferrosilicon or aluminum phosphates. They may be admixed with the aqueous alkali silicate either in dissolved or in suspended form, or sprayed later onto the free-flowing product. Further, the perlite may be either first sprayed with up to 20% by weight of water and then mixed with an insoluble hardener before the mixture of aqueous alkali silicates and hydrophobizing agent is applied, or else the perlite is sprayed with the dissolved or suspended hardener before it is mixed with the mixture of aqueous alkali silicates and hydrophobizing agent. Finally, the free-flowing product may also be admixed with a dry powdery hardener.
The amounts of the aqueous alkali silicate and added hydrophobizing agent and optionally the hardeners are only so large, on principle, that the perlite still remains a free-flowing bulk material. Higher amounts of aqueous alkali silicates and additional water, which result in a clotting or aggregation of the perlite or even castability of the mixture, are to be avoided. Such materials are not or but difficult to handle, and can hardly be filled in molds, or they contain an unnecessarily high amount of water. Removal of the latter consumes more energy and is therefore undesirable.
To increase the tensile strength, the light-weight material according to the invention may be provided with high-tensile strength cover layers on one or more surfaces thereof. There may be used, in particular, fiber glass fabrics, fiber glass webs, fiber glass strips, mats or strips of all kinds, such as metal foils, which may also be perforated, as well as metal gauzes or metal strips, all of which may be open to water vapor diffusion or water-vapor impermeable, but always have a high tensile strength. They are applied either already in the preparation of the molded light-weight materials by covering the bottom and/or the side walls of the molds with the high-tensile strength materials prior to filling them with the free-flowing bulk material, said materials having previously been brushed, soaked or sprayed with a binder, preferably aqueous alkali silicates. If desired, such a layer may also be placed on top after filling the molds, prior to the mold""s being optionally slightly compressed and then heated. It is altogether possible as well to adhere such high-tensile strength layers later to surfaces left uncovered in the first production step. However, this usually requires a second heating step.
The process for producing the light-weight material according to the invention is generally performed in such a way that the expanded perlites having a grain size of from 0.8 to 6 mm are mixed with a mixture of aqueous alkali silicates and the hydrophobizing agent and optionally soluble or suspended hardeners to give still free-flowing products, the mixture is filled in molds, optionally slightly compressed and then heated.
If hardeners for aqueous alkali silicates are to be employed, they are preferably sprayed onto the free-flowing product as a solution or suspension. However, it is also possible to add such hardeners directly to the mixture of aqueous alkali silicate and hydrophobizing agent, unless this will result in too quick a reaction with the alkali silicates. Especially insoluble hardeners are preferably adhered to wetted perlite, which is previously wetted with up to 20% by weight of water, based on the weight of the perlite. This wetted perlite is then mixed with a mixture of aqueous alkali silicate and hydrophobizing agent.
The heating of the filled and optionally slightly compressed molds is effected by microwaves since this results in a particularly uniform heating of the inner regions as well. In addition, it has been found that heating by microwaves yields a strength which is by up to 400% higher than when heated in an oven.
After filling the material in molds, the final volume is preferably reduced by gently pressing or compressing the mold. Depending on the selected charged weight and the selected final volume, the bulk density of the light-weight material may also be influenced in this way. Too hard pressing results in bulk densities higher than 500 kg/m3.
If the shaping is supported by pressing, the molded parts may be demolded prior to heating and conveyed through the microwave plant in this form. It may be convenient to leave the molded part on a microwave-transparent bottom plate which is conveyed along through the passage plant or belt plant. When belt plants are employed, the molded part is dried by moving to and fro through the microwave zone.
In addition to the bottom side and top side of the molded part, if desired, its side edges may also be lined with a high-tensile strength material, after having been brushed, soaked or sprayed with a binder, such as aqueous alkali silicate.
One great advantage of the light-weight materials according to the invention is the fact that the heating mainly serves to firmly bond the perlite grains together, but it is not necessary to remove major quantities of water by evaporation. The latter is a very substantial cost factor and also contributes to a rather energy-saving production of the material.
The light-weight materials according to the invention generally consist of from 50 to 500 mass parts of expanded perlite, 100 mass parts of aqueous alkali silicate, and from 2.0 to 18.5 mass parts of hydrophobizing agent. If hardeners are added, this is done in amounts of from 0 to 100 mass parts.
The light-weight material according to the invention has very high adhesive, tensile, tensile-in-bending and compressive strengths and a good heat insulation, as related to its bulk density. It contains little or no combustible materials and only contains safe components, in terms of health hazards. The light-weight material may be open to water vapor diffusion. However, if desired, it may also be provided with water-vapor impermeable outer layers. Its open-pore structure results in a good acoustic absorption. The bulk hydrophobization results in overall hydrophobic properties, also at fractures and cutting edges.